1. Field of the Invention
This invention relates to a thermal mass flowmeter and controller which includes sensors for measuring the mass flow rate of liquid or gaseous fluids, electromagnetic control valves, associated amplifiers, and bridge and signal conditioning circuits. It further relates to the utilization of semiconductor type materials to make extremely small and accurate flowmeters for measuring and controlling the flow of semiconductor processing gases, medical anesthetic gases, and gases used for analytic instruments.
2. Prior Art
In addition to the prior art recited in the related application, namely single element hot wire anemometers and flow sensors utilizing two or three-element configurations of heaters and temperature sensors, U.S. Pat. No. 4,478,076 shows a flow sensor consisting of a pair of heat sensors disposed on opposite sides of a heater. The sensors and heaters are fabricated by depositing resistive material on a thin film of silicon nitride previously deposited on a silicon substrate. After patterning the resistor it may be encapsulated with a suitable thermal insulator such as silicon nitride. An air space etched out underneath the sensor elements allows air flow past the top and bottom of the sensors and heater.
U.S. Pat. No. 4,471,647 describes in context of a gas chromatographic assembly, a thermal detector in which the gas being thermally measured passes through apertures in a silicon dioxide membrane containing sensor and heater elements. The membrane-sensor structure is supported by a silicon substrate in which an aperture is etched to thermally isolate the membrane, provide perimeter support, and allow gases to pass through the substrate.
U.S. Pat. No. 3,931,736 shows an improvement over fluid flow sensors which are positioned on the exterior surface of a conduit. The improvement entails placing a thin imperforate membrane functioning as one wall of an active flow channel and a dead flow channel, the latter to obtain reference readings under conditions of substantially no fluid flow. Sensors are placed on the surface of the membrane away from the channels so that the sensors are not in direct contact with the fluid. The bottom surface of the membrane is in contact with the fluid and the sensor chips are thermally coupled to the membrane.
U.S. Pat. No. 4,343,768 describes a catalytic gas detector formed from a self-heated temperature sensor deposited on a thin dielectric film supported by a substrate. The substrate is etched under the sensor region to form a physical bridge which is thermally isolated. Catalytic films are deposited on the heater-sensor to promote chemical reactions. The sensor measures heat generated by the reaction.
The above designs can be used to measure the mass flow rate of fluids passing parallel to and near the substrate surfaces. They have limited inherent accuracy because the sensor elements may not be placed at a location within the flow conduit where the local fluid velocity is representative of the total flow, and the substrate placement in the flow conduit may introduce turbulence--further decreasing measurement accuracy.